Top is the door as a base layer (straightened with the perspective tool)
The paint splash extracted from the image and scaled (80%) then pasted into a transparent layer.
(important to have the layers the same size otherwise you get into a mess with the bump map offsets)

Bottom image is the bump map dialog, note you are bump mapping the paint layer and using the door as the bump map. This is the default setting, for any image you need to look at say light direction and adjust accordingly. Just play around with the settings and see what results.

Top image - you should get the paint layer impressed with the door layer contour.
Erase bits that would be under the grill, door handle etc.
Small areas that might not respond to the bump map try and deform them a little with the cage tool (not my favourite gimp tool by a large margin, but ok for small areas.)

Then it depends how much time you want to spend on your masterpiece. I would give the whole of the paint layer a small (2 pix) gaussian blur. Bits around the metalwork need a bit of effort.

A very quick trial to get the above screenshots. Missed the bit at the door sill for example.lol

You can use any of the brushes with the eraser tool.
It is much the same as the paint tool.
- resize the brush either with the slider or enter a size directly or use the square brackets ][ to step the size up and down.
- Erase in straight lines. Click on a start point, hold the shift key down, move to the end point, click and it erases in between. Very useful with a fuzzy brush but check on the brush spacing.

I do not think you will ever get it totally realistic, as before lots of tweaking, depends how much effort you put into it.

So here as well as erasing the grill.
bump mapping
blurring
embossing
lighting effects
dodge tool on selected bits .. and so on.

By the time you do all that, quicker to go and throw some paint on the door and take a new photograph.